The present invention relates generally to contact lens design, and in particular to a method for defining a contact lens surface.
The surface topographies of normal human corneas are not spherical. Instead, their surfaces flatten at varying and unpredictable rates from the center of the cornea to the periphery. The fit of contact lenses that rest entirely on the cornea must take this corneal shape factor into account. For example, the tolerance of rigid contact lenses is largely dependent on their ability to slide over the cornea in a vertical motion with each blink. This provides a mechanism of pumping fresh tears under the lens that brings with it oxygen and flushes away accumulated debris and waste products. It is well known that if a spherical rigid contact lens is placed on a cornea, any movement would cause the edge of the lens to dig into the flatter surface of the peripheral cornea. This would curtail lens movement and traumatize the cornea. To avoid this problem, the peripheral surfaces of rigid contact lenses are made flatter than their centers so that they form a ski-like relationship with the cornea, for reasons including, for example, the flushing of accumulated debris and waste products. While the lens design must provide sufficient edge clearance, at the same time the thickness of the lens edge must permit the eye lid to slide over the lens without irritation to the eye lid surface. It is desirable that a rigid lens slide over the cornea in a vertical motion with each blink of the eye. Lens position relative to the cornea and the amount and consistency of blink-induced lens movement are also considered for the wearer's comfort.
An essential characteristic of the lens design is the creation of a space between the edge of the lens and the underlying cornea, known as edge clearance or edge lift. Although it must be sufficient to keep the edge of the lens above the surface of the cornea during lens movement, the edge clearance should also be as small as possible to avoid irritating the edge of the upper lid so it slides over the edge of the lens during the closing phase of a blink. As the lens shifts position on the eye during the blinking process, the edge clearance acts as a scoop to bring a fresh supply of tear fluid under the lens and alleviate the problem of oxygen depletion. A lens made of gas permeable plastics, which allow direct transfer of oxygen through the material itself, also assists in this process but some form of edge lift is still desirable. As a result, the precise amount of edge lift can be critical in determining the wearing comfort of a rigid contact lenses.
Other factors important to rigid contact lens wearing comfort are the amount and consistency of blink-induced lens movement and lens position since a lens that positions higher on the cornea will be more comfortable than one that positions low. Both of these lens fitting characteristics are influenced by the width and shape of the peripheral contact lens fitting zone which, ideally, should be tailored to the topography of the individual peripheral cornea.
The typical approach to creating a flatter peripheral lens surface and adequate edge clearance has been to generate a series of conic section curves, each having a radius of curvature larger (flatter) than the preceding one. If these curves are spherical, they will create sharp junctions where they join and these junctions must be removed by hand polishing to avoid irritating the cornea. The method has serious limitations. The blending of the curves is subject to human judgment and operator skill, and it may be incomplete resulting in contact lens wearing discomfort or it may be excessive and thereby create a different unpredictable surface shape. The result is a lack of reproduce ability and precision when fabricating the peripheral surface of rigid contact lenses.
Another method of designing a junctionless posterior contact lens surface is to choose a series of progressively flatter conic section curves, such as ellipses, that join tangentially. However, the requirement of tangential joining greatly limits the choice of curves and such curves lack the flexibility of being configured to have a precise relationship to a specific corneal shape.
Thus, when a contact lens is placed onto the cornea, a peripheral portion of the back surface of the lens makes primary contact with the eye. The central optical portion of the contact lens generally vaults the central cornea and is supported by the periphery.
Another aspect of lens design is the central portion of a lens that provides the optics. The central portion is generally described by a conic section (usually a sphere or ellipse). Any other type of optical surface has been very difficult and costly to manufacture, although using computer controlled manufacturing equipment, optical surfaces may deviate from the traditional regular conic. An object of this invention is lens manufacture using the advantages of computer controlled manufacturing equipment whether or not the central portion is described by a conic section. Another object of the invention is to manufacture the front and back surfaces of the lens without creating any junctions that require human polishing or blending.
An object of this invention is a novel method to efficiently manufacture a smooth and reproducible lens surface that fits the cornea. Accordingly, another object of the invention is to permit the fitter control over the shape and magnitude of edge clearance, particularly axial edge clearance.